Conductive material dispersion, and electrode and lithium secondary battery manufactured using the same

This application is a continuation of U.S. application Ser. No. 18/377,994 (now U.S. Pat. No. 12,218,359), filed on Oct. 9, 2023, which is a continuation of U.S. application Ser. No. 17/604,084 (now U.S. Pat. No. 11,824,200), filed on Oct. 15, 2021, which is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/KR2020/006238, filed on May 12, 2020, which claims the benefit of Korean Patent Application No. 10 2019-0057722, filed on May 17, 2019, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in its entirety by reference.

The present invention relates to a conductive material dispersion, and an electrode and a lithium secondary battery manufactured using the dispersion. More specifically, the present invention relates to a conductive material dispersion and an electrode and a lithium secondary battery manufactured using the same, wherein the conductive material dispersion has low viscosity properties by using a copolymer including a specific unit as an auxiliary dispersant together with a nitrile-based copolymer, which is a main dispersant.

A secondary battery is a battery which may be repeatedly used through a discharging process in which chemical energy is converted into electrical energy and a charging process in the reverse direction thereof. The secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator, and in general, the positive electrode and the negative electrode are composed of an electrode current collector and an electrode active material layer formed on the electrode current collector. The electrode active material layer is prepared by applying an electrode slurry composition including an electrode active material, a conductive material, a binder, and the like on the electrode current collector, followed by drying, and then roll-pressing.

A conductive material is to improve the conductivity of the electrode active material, and fine carbon materials such as carbon black, ketjen black, fullerene, graphene, carbon nanotube (CNT), and the like are mainly used as the conductive material.

However, since conductive materials of carbon materials are easily aggregated without being uniformly dispersed in an electrode slurry composition, when an electrode is manufactured by using the same, a conductive material is not evenly distributed in an electrode active material layer. In order to overcome such a limitation, a method has been recently developed in which a conductive material is first mixed with a dispersant such as PolyVinyl Pyrrolidone (hereinafter, PVP), acrylonitrile-butadiene rubber, and the like in a solvent to prepare a conductive material dispersion, and then the conductive material dispersion is applied to an electrode slurry composition.

However, since the viscosity of a conductive material dispersion using a PVP dispersant increases rapidly when the content of a conductive material increases, there is a limit to increasing the content of the conductive material, and thus, there is a limit to improving electrical conductivity.

Meanwhile, in order to lower the viscosity of a conductive material dispersion, it is preferable to use a dispersant having a low weight average molecular weight. However, in the case of an acrylonitrile-butadiene rubber dispersant, there is a problem in that the storage stability thereof rapidly degrades due to gelation when the weight average molecular weight is lowered.

Meanwhile, the solid content of a positive electrode slurry is determined according to the solid amount of a conductive material dispersion, and when the solid content of a positive electrode slurry is high, there are effects such as the increase in productivity, improvement in electrode drying efficiency and binder migration, improvement in adhesion force, and the like. Therefore, it is preferable to increase the solid content of the conductive material dispersion, but the viscosity increases when the solids content increases, and thus, there is a problem in processability.

Therefore, there is a demand for developing a conductive material dispersion having low viscosity properties even when the conductive material content is high.

An aspect of the present invention provides a conductive material dispersion having low viscosity properties compared with an existing conductive dispersion by using a copolymer including a specific unit as an auxiliary dispersant together with a nitrile-based copolymer dispersant in the conductive material dispersion.

Another aspect of the present invention provides an electrode and a lithium secondary battery manufactured using the conductive material dispersion.

According to an aspect of the present invention, there is provided a conductive material dispersion including a carbon-based conductive material, a dispersant, and a dispersion medium, wherein the dispersant includes a main dispersant and an auxiliary dispersant, and the main dispersant is a nitrile-based copolymer and the auxiliary dispersant is a copolymer including an oxyalkylene unit and at least one selected from the group consisting of a styrene unit and an alkylene unit.

According to another aspect of the present invention, there is provided an electrode including an electrode active material layer formed of an electrode slurry composition containing an electrode active material, the conductive material dispersion, a binder, and a solvent. At this time, the electrode may be a positive electrode.

According to yet another aspect of the present invention, there is provided a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. At this time, the positive electrode may be the electrode according to the present invention.

A conductive material dispersion according to the present invention uses a nitrile-based copolymer together with a copolymer including an oxyalkylene unit and a styrene unit and/or an alkylene unit, and thus, has lower viscosity properties than a conductive material dispersion using a nitrile-based copolymer dispersant alone. Accordingly, the solid content in the conductive material dispersion may be increased compared to an existing conductive dispersion, and as a result, when manufacturing an electrode, there may be effects such as the increase in productivity, improvement in electrode drying efficiency and binder migration, improvement in adhesion force, and the like.

It will be understood that words or terms used in the specification and claims of the present invention shall not be construed as being limited to having the meaning defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having meanings that are consistent with their meanings in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention.

In the present specification, the terms “include,” “comprise,” or “have” are intended to specify the presence of stated features, numbers, steps, elements, or combinations thereof, and do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

In the present specification, “specific surface area” is measured by a BET method, and specifically, may be calculated from the adsorption amount of nitrogen gas under a liquid nitrogen temperature (77K) using Belsorp-mino II of BEL Japan Co.

In the present specification, a “weight average molecular weight (Mw)” refers to a conversion value for a standard polystyrene measured by Gel Permeation Chromatography (GPC). Specifically, the weight average molecular weight is a value converted from a value measured under the conditions below using GPC, and a standard polystyrene of the Agilent system was used for making a calibration curve.

<Measurement Conditions>

Hereinafter, the present invention will be described in more detail.

Conductive Material Dispersion

First, a conductive material dispersion according to the present invention will be described.

The conductive material dispersion according to the present invention includes a carbon-based conductive material, a dispersant, and a dispersion medium. At this time, the dispersant includes a main dispersant and an auxiliary dispersant, and the main dispersant is a nitrile-based copolymer and the auxiliary dispersant is a copolymer including an oxyalkylene unit and at least one selected from the group consisting of a styrene unit and an aliphatic hydrocarbon unit.

Hereinafter, each component of the conductive material dispersion according to the present invention will be described in detail.

(1) Carbon-Based Conductive Material

The carbon-based conductive material is to improve the conductivity of an electrode, and a carbon-based conductive material commonly used in the art, for example, a carbon nanutube, carbon black, or the like, may be used.

In the carbon nanotube, a graphite sheet has a cylindrical shape of a nano-sized diameter and has an sp2 bonding structure, and exhibits conductor or semiconductor properties depending on the angle and structure at which the graphite surface is rolled. The carbon nanotube may be classified as a single-walled carbon nanotube (SWCNT), a double-walled carbon nanotube (DWCNT), and a multi-walled carbon nanotube (MWCNT) depending on the number of bonds forming a wall. Such a carbon nanotube may be appropriately selected according to the use of the dispersion.

In addition, the carbon nanotube may have a secondary shape in which a plurality of carbon nanotubes are aggregated or arranged. For example, the carbon nanotube may be a bundle-type carbon nanotube in the form of a bundle or a rope in which or a plurality of carbon nanotubes are arranged or aligned in parallel in a predetermined direction, or may be an entangled-type carbon nanonube in the form of a sphere or a potato in which a plurality of carbon nanotubes are entangled without a certain directionality. In terms of dispersibility, it is more preferable that the carbon nanotube is a bundle-type carbon nanonube.

Meanwhile, as the carbon black, commercially available furnace black, channel black, thermal black, acetylene black, ketjen black, hollow carbon black, or the like may be used. The type of the carbon nanonube is not particularly limited.

The carbon black may have been, as needed, surface-treated by a method known in the art. For example, the carbon black may have been surface-treated by acetylene gas, and thus, free of impurities. In addition, the carbon black may have a purity of 99.5% or greater.

Meanwhile, the carbon-based conductive material used in the present invention may have a BET specific surface area of 1000 m/g or less, preferably 30 to 1000 m/g. When the BET specific surface area of the carbon-based conductive material is greater than 1000 m/g, dispersion may not be smoothly achieved.

Specifically, when the carbon-based conductive material is a carbon nanonube, the BET specific surface area of the carbon nanonube may be 100 to 1000 m/g, 150 to 800 m/g, 150 to 500 m/g, 150 to 300 m/g, or 150 to 200 m/g.

When the carbon-based conductive material is carbon black, the BET specific surface area of the carbon black may be 30 to 1000 m/g, preferably 30 to 400 m/g, more preferably 30 to 380 m/g, even more preferably 30 to 150 m/g.

Meanwhile, the content of the carbon-based conductive material in the conductive material dispersion may be 0.1 to 30 wt %, preferably 1 to 30 wt %. Specifically, when the carbon-based conductive material is a carbon nanonube, the content of the carbon-based conductive material in the conductive material dispersion may be 0.1 to 10 wt %, preferably 1 to 8 wt %, and when the carbon-based conductive material is carbon black, the content of the carbon-based conductive material in the conductive material dispersion may be 1 to 30 wt %, preferably 1 to 25 wt %. When the content of the carbon-based conductive material is too low, there may be problems in that a loading amount is reduced during the manufacturing of an electrode, so that process cost increases, and binder migration occurs during the manufacturing of the electrode, so that adhesion force is reduced. Meanwhile, when the content of the carbon-based conductive material is too high, there is a problem in that the viscosity of the conductive material dispersion increases.

(2) Dispersant

A conductive material dispersant according to the present invention two includes kinds of dispersants. Specifically, the conductive material dispersant according to the present invention includes a nitrile-based copolymer as a main dispersant and as an auxiliary dispersant, a copolymer including an oxyalkylene unit and at least one selected from the group consisting of a styrene unit and an aliphatic hydrocarbon unit.

2-1) Main Dispersant

The main dispersant is to improve the conductive material dispersibility in a conductive material dispersion, and may be, specifically, a copolymer having a α,β-unsaturated nitrile-derived unit and a conjugated diene-derived unit. At this time, the conjugated diene-derived unit may be partially or fully hydrogenated. A method for subjecting the conjugated diene-derived unit to hydrogenation may be performed by a hydrogenation method known in the art, for example, by a catalytic hydrogenation reaction using a catalyst system such as Rh, Ru, Pd, and IR, and the hydrogenation rate may be adjusted by adjusting the amount of catalyst, reaction hydrogen pressure, reaction time, and the like.

The nitrile-based copolymer may be prepared by copolymerizing an α,β-unsaturated nitrile monomer and a conjugated diene-based monomer, and then hydrogenating a C═C double bond in the copolymer. The polymerization reaction and hydrogenation process of the monomers may be performed according to a typical method known in the art.

As the α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile may be used, and any one thereof or a mixture of two or more thereof may be used.

As the conjugated diene-based monomer, for example, conjugated diene-based monomers having 4 to 6 carbon atoms such as 1,3-butadiene, isoprene, 2,3-methyl butadiene, or the like may be used, and any one thereof or a mixture of two or more thereof may be used.

Meanwhile, the nitrile-based copolymer may include a α,β-unsaturated nitrile-derived unit: a conjugated diene-derived unit at a weight ratio of 10 to 50:50 to 90, preferably 20 to 40:60 to 80, and more preferably 25 to 40:60 to 75. When the content of each unit in the nitrile-based copolymer satisfies the above range, dispersibility and high-temperature properties are excellent. Here, the content of the α,β-unsaturated nitrile-derived unit may be the median value of a value to be quantified by measuring the amount of nitrogen generated in accordance with a mill oven method of JIS K 6364 and calculating its binding amount from the molecular weight of α,β-unsaturated nitrile. The content of the conjugated diene-derived unit may be a value obtained by subtracting the weight of the α,β-unsaturated nitrile-derived unit from the weight of all copolymers.

Meanwhile, the nitrile-based copolymer of the present invention may have a hydrogenation rate of the conjugated diene-derived unit of 80% or greater, preferably 90%. This is because, when dispersant a having unhydrogenated conjugated diene units is used, the reactivity with an electrolyte solution may increase due to double bonds in conjugated diene, so that high-temperature properties may deteriorate.

According to one embodiment, the nitrile-based copolymer may include a repeating unit represented by [Formula 1] below and a repeating unit represented by [Formula 2] below.

At this time, the content of the repeating unit represented by [Formula 1] above may be 10 to 50 wt %, preferably 20 to 40 wt %, and more preferably 25 to 40 wt %, and the content of the repeating unit represented by [Formula 2] above may be 50 to 90 wt %, preferably 60 to 80 wt %, and more preferably 60 to 75 wt %.

Meanwhile, the weight average molecular weight of the main dispersant may be 10,000 to 500,000 g/mol, preferably 20,000 to 400,000 g/mol, more preferably 20,000 to 300,000 g/mol, and yet more preferably 20,000 to 100,000 g/mol. When the weight average molecular weight of the main dispersant satisfies the above range, a conductive material may be uniformly dispersed with a small amount of dispersant, and the solution viscosity be prevented from being excessively increased when dispersing the conductive material, which is advantageous in processing.

2-2) Auxiliary Dispersant